Known processes for producing tetrafluorophthalic acid include (1) a process of fluorinating tetrachlorophthalonitrile and hydrolyzing tetrafluorophthalonitrile, as described in Kogyo Kagaku Zasshi (Journal of Industrial Chemistry), 72, 447-448 (1970); and (2) a process of chlorinating tetrachlorophthalic anhydride, fluorinating the tetrachlorophthalic acid dichloride thus obtained, and then hydrolyzing the tetraflourophthalic acid difluoride, as described in Zh. Obshabh. Khim., 36, 139 (1966).
However, the first process described above has the disadvantage that the phthalonitrile which is used as the starting material is hard to handle due to its high toxicity. Furthermore, the polymerization of phthalonitrile occurs during the synthesis of tetrachlorophthalonitrile by the vapor phase chlorination reaction of phthalonitrile, which reduces the yield and frequently inactivates the catalyst. For these reasons, it is difficult to use this process for the industrial production of tetrafluorophthalic acid. According to the description of Organic Chlorine Compounds, pages 490-493 (John Wiley Co., 1948), the second process described above is an uneconomical process, since tetrachlorophthalic acid dichloride is easily isomerized to form 3,3,4,5,6,7-hexachlorophthalide; the yield for the fluorination reaction of tetrachlorophthalic acid dichloride is low because of the isomerization reaction; and further the molar equivalent of the fluorination reagent required is 6 times that of tetrachlorophthalic acid dichloride.
The above-described processes have various disadvantages that prevent their practical use in industrial production processes.